Many foodstuffs, such as fresh produce (e.g., strawberries, lettuce, tomatoes, cauliflower), can have their fresh appearance maintained for a substantial period of time by controlling the gaseous environment of the package within which they are contained. In this way, losses during shipment and storage can be significantly reduced and thereby insuring that a larger percentage of fresh produce can be delivered to the customer in acceptable purchasable condition.
Many kinds of fresh produce, a primary one being strawberries, have been shipped in the past by storing a quantity of such boxes on a pallet base, with the entire load being enclosed by a plastic bag that is sealed to the base. U.S. Pat. No. 4,055,931, discloses the providing of a special atmosphere into the containing space formed by the plastic bag by first pulling a vacuum on the bag, then inserting a sharp-ended nozzle through the plastic sheeting and adding pressurized gases to the interior on a timed basis.
As fresh produce ripens, it gives off certain gases which tend, by themselves, if the package is totally sealed to provide a continuing change of the gaseous content of the package. In the past, to compensate for the production of gases by the aging of the produce, plastic bags have been made of sheet plastic that will permit migration of, say, oxygen or other selected gaseous molecules through the plastic bag and in that way prevent the internal accumulation of that gas to an undesirable high level. However, in order to control the interchange of a gas through the plastic envelope of the produce package, it is important that there be no macroscopic leaking of gas through tears in the bag wall or openings between the bag sheeting and the pallet base. That is, for this approach to work on a practical scale, it is important that edges of the plastic sheeting be sealed to the base with no gaps or interstices that would permit relatively large amounts of gas to leak therepast.
Various approaches have been taken in the past to securing the plastic bag over the produce and onto the pallet base, but all have been found unsatisfactory in one respect or another. Hand taping, where workmen pull off a length of tape and place it onto the lower edge of the plastic sheeting and also onto the base itself, is just too unreliable for a quantity production process. Specifically, hand-taping tends to produce an unacceptably high incidence of tape wrinkling and partial adhesion of the tape to plastic bag and pallet base via which gas can escape. Other sealing techniques such as one-sided heat sealing and impulse heat sealing have been attempted and found impractical from a cost and efficiency standpoint, and failure to provide an atmospheric seal under commercial conditions.